Field of the Invention
The present invention relates to a current-to-voltage conversion circuit and a self-oscillation circuit.
Priority is claimed on Japanese Patent Application No. 2014-107185, filed on May 23, 2014, the contents of which are incorporated herein by reference.
Description of Related Art
Japanese Unexamined Patent Application, First Publication No. 2006-71498 (hereinafter, referred to as “PTL1”) discloses a circuit for converting capacitance changes of an oscillator into voltage changes and outputting the voltage changes. According to the technique disclosed in the PTL 1, a conversion circuit, which includes an operation amplifier, a resistance, and a capacitor, converts the capacitance changes of the oscillator into the voltage changes.
Japanese Examined Patent Publication No. 46-27633 (hereinafter, referred to as “PTL2”) discloses a circuit for converting current changes into voltage changes using a capacitor and a transistor including a grounded base.
In the technique disclosed in the PTL 1, an input offset voltage is occurred. The input offset voltage is obtained by converting a voltage (output offset), which is occurred in an output of an operation amplifier when the input into the operation amplifier is zero, into an input value. The input offset voltage changes depending on the influence of temperature, aging, flicker noise, which is certainly decreased as the frequency increases, and the like. When the input offset voltage changes, the errors of the frequency and the amplitude of the output voltage value are occurred during the changing of the voltage applied to the oscillator, which is converted by the operation amplifier. In addition, according to the technique disclosed in the PTL 1, consumption current is occurred for driving the operation amplifier. If the signal-to-noise ratio (SN ratio) of the operation amplifier is improved or the band of the operation amplifier is enlarged, this consumption current is generally increased.
According to the technique disclosed in the PTL 2, compared to the case where an operation amplifier is used, there are some cases where the consumption current is decreased using the transistor. In this case, according to the technique disclosed in the PTL 2, it is necessary to switch on the transistor, which is for executing conversion to obtain output voltage, and to pass the current into a collector of the transistor. However, since the voltage difference between a base and an emitter in the transistor, which is in the range from about 0.3V to about 0.6V, is required, there are some cases where minimal input signals cannot be converted. In addition, according to the technique disclosed in the PTL 2, if the size of the input current changes and the voltage difference between the base and the emitter in the transistor is equal to or less than 0.3V, the operation of the current conversion is stopped. Therefore, there are some cases where the distortion of the waveform of the current output from the transistor is occurred. In addition, according to the technique disclosed in the PTL 2, since the capacitor is used to detect the current, the stabilization of the electrical potential applied to a terminal of the capacitor, which is connected to the transistor, requires long period of time. Therefore, if the output current changes of the oscillator are converted into the voltage changes, there are some cases where the above-stated situations are occurred.